Aforesaid processes and devices, particularly for determining the percentage limiting value of a brake fluid, are known which are based on the established fact that by increasing the water content in the brake fluid the boiling point of this fluid or liquid mixture is reduced, and thus represents a measurement for the portion of the water contained in the brake fluid.
With the known processes, a sample is always taken from the liquid and heated in a test chamber, until vaporization takes place in connection with a temperature measurement and an admission check of the test chamber, so as to obtain the temperature measurement as a measurement for the boiling point when the fluid is displaced by the steam from the test chamber (DE-OS No. 27 21 732).
With a different process (EU-OS No. 0 056 424), a sample is also taken from the liquid by means of a measuring probe and heated whereby the temperature and the temperature change in time of the heating element located in the sample is measured, and the temperature reached during a temperature change in time is registered as a measurement for the boiling point of the brake fluid.
These known processes require sample taking for each measurement, the samples being separated from the greater part of the liquid mixture and returned to it after completion of the measurement. The separation of the sample, as well as its transfer to a pressure chamber, and its heating up to a specific steam volume in the pressure chamber, which is required for each measurement, is costly and relatively complicated, and requires a relatively long time interval for heating and cooling of the sample.